1. Field of the Invention
The present invention relates to a liquid-crystal display device.
2. Description of the Related Art
A liquid-crystal display device (hereinafter, also referred to as LCD) has been used in a broadening range of fields every year as an image display device which has low power consumption and saves spaces. A liquid-crystal display device has a constitution in which a backlight (hereinafter, also referred to as BL), a backlight-side polarizing plate, a liquid crystal cell, a display-side polarizing plate, and the like are provided in this order.
Recently, for liquid-crystal display devices, development for power saving, high definition, and color reproducibility improvement has been underway in order to improve LCD performance. At the moment, while there is a significant demand for power saving, high definition, and color reproducibility improvement particularly in small-sized display devices such as table PCs and smartphones, development of next-generation Hi-visions (4K2K, EBU ratio of 100% or higher) with current TV standards (FHD, 72% of National Television System Committee (NTSC) ratio≈100% of European Broadcasting Union (EBU) ratio) is also underway for large-sized display devices. Therefore, there is an intensifying demand for power saving, high definition, and color reproducibility improvement in liquid-crystal display devices.
In accordance with power saving in the backlight, there are cases in which an optical sheet member having a luminance-improving function is provided between the backlight and the backlight-side polarizing plate. This optical sheet member is an optical element that transmits only light rays vibrating in a specific polarization direction among incidence light rays vibrating in random directions and reflects light rays vibrating in other polarization directions. As a core member of a low-power LCD developed in response to an increase in mobile devices and a decrease in the power consumption of home appliance, it is expected to increase luminance (the degree of brightness of a light source per unit area) by solving the low optical efficiency of LCDs.
As the above-described member, a technique is known in which a specific optical sheet member (Dual Brightness Enhancement Film (registered trademark, DBEF) or the like) is combined between the backlight and the backlight-side polarizing plate so as to improve the light utilization ratio of the BL by means of light recycling and thus the luminance of the BL is improved while saving power in the backlight (refer to JP3448626B). Similarly, JP1989-133003A (JP-H1-133003A) describes a technique in which the light utilization ratio of the BL is improved by means of light recycling by broadening the bandwidths in a polarizing plate obtained by laminating a λ/4 plate and a cholesteric liquid crystalline phase and in a layer formed by fixing three or more layers of a cholesteric liquid crystalline phase having different pitches between the cholesteric liquid crystalline phases.
However, the above-described optical sheet member has a complicated member constitution, and thus, in order to distribute the optical sheet member in the market, it becomes essential to reduce the cost by decreasing the number of members by means of additional integration of functions of the members.
Meanwhile, from the viewpoint of high definition and color reproducibility improvement of a liquid-crystal display device, a method for sharpening the light emission spectrum of the backlight is also known. For example, JP2012-169271A describes a method for increasing luminance and improving color reproducibility by realizing white light using a quantum dot (QD) which emits red light and green light as a fluorescent body between a blue LED and a light guide plate. In SID'12 DIGEST p. 895, a method of combining a light conversion sheet (QDEF, also referred to as quantum dot sheet) in which a quantum dot is used in order to improve the color reproducibility of the LCD is proposed.